1. Field of the Invention
The invention relates to a method for preparing polymer electrolytes for rechargeable lithium intercalation cells which contain a solution of a dissociable lithium salt dispersed in a polymeric maxtrix.
2. Description of the Related Art
Polymer electrolytes, methods for their preparation, and their use are disclosed by U.S. Pat. Nos. U.S. Pat. No. 5,296,318, U.S. Pat. No. 5,418,091 and U.S. Pat. No. 5,429,891. The polymer electrolytes obtained in accordance with the known methods can be used as a separator in secondary lithium cells, the material at the same time assuming the function of the electrolyte. The solid polymer electrolytes (SPE) thus represent a combination of separator and immobilized electrolyte. These can be used, in particular, in lithium intercalation cells, also known as lithium-swing or rocking-chair cells. Compared with other separators, the solid polymer electrolytes, owing to their electrolyte content and only slight tendency to flow, are particularly suitable for the construction of galvanic cells whose electrode and separator thicknesses are in the range of from 80 to 250 .mu.m.
According to the known methods the plasticizers used in the preparation of polymer electrolytes are exchanged for a solution of the dissociable lithium salt such as LiPF.sub.6, LiClO.sub.4 or LiBF.sub.4 (also referred to as conducting salts) in organic solvent mixtures such as ethylene carbonate (EC)/dimethyl carbonate (DMC), propylene carbonate (PC)/diethyl carbonate (DEC), dimethoxyethane (DME) or dipropyl carbonate (DPC). This preferably involves extraction of the plasticizers by solvents in which the starting polymer is insoluble, such as diethyl ether, hexane, Freon 113 or methanol. The polymer film is then dried. Impregnation of the polymer film with a solution of the dissociable lithium salt can take place both separately and after lamination of the electrodes with a polymer electrolyte.
It is further known to use, as the plasticizer, a mixture of the abovementioned solvents and conducting salts dissolved therein, said mixture being referred to as a liquid electrolyte and being trapped within the polymer matrix being formed.